The utility of KDP (potassium deuterium phosphate) and lithium tantalate crystals in modulating light beams has long been known. The electrically responsive birefringence of these crystal materials enables a wide range of light beam modulation; however, the temperature-responsive natural birefringence which these crystals also exhibit detracts significantly from the modulated signal stability necessary in many applications.
The lithium tantalate crystal is particularly desirable for use in an electrooptic modulator, since it is substantially non-hygroscopic, has a moderately high electrooptic coefficient, and relatively low degree of birefringence. This latter property, although low, is sufficient to significantly detract from the utility of a modulator unless the affects of temperature variations are controlled. In this respect a useful means of significantly reducing or eliminating the effect of temperature changes on the natural birefringence of the lithium tantalate crystal modulator has been known for some time and entails the arrangement of a pair of such crystals in a light beam in such a manner that their optic axes are effectively orthogonal to one another. The effect of such an arrangement is to cause the temperature-responsive natural birefringence of the assembled lithium tantalate crystal pair to cancel.
This practice of accounting for temperature-responsive variations in the natural birefringence of lithium tantalate crystals is described in an article by M. R. Biazzo, "Fabrication of a Lithium Tantalate Temperature-Stabilized Optical Modulator", APPLIED OPTICS, Vol. 10 No. 5, pg. 1016 (1971). While the basic approach described in that article for the construction of a modulator has proven useful to some extent the procedure does not lend itself to the practical manufacture of optical modulators at reasonable cost.
Although the natural, temperature-responsive birefringence of a pair of lithium tantalate crystal elements may be matched to a significant degree by cutting them from a single boule, or mother crystal, such elements, in fact, possess some significant variations in this property which can adversely affect the resulting modulator. Since such variations do not become apparent until final assembly and testing of the modulator described, for example, by Biazzo, substantial and costly manufacturing time is expended in the search for a properly matched pair of crystal elements.
Such limitations to the practical utility of lithium tantalate crystals in the manufacture of light beam modulators are obviated by the present invention in which single-crystal subassemblies are constructed in such a manner as to enable the testing of various combinations of crystals in order to identify perfectly matched pairs prior to any final assembly of a modulator. As a result, costly disassembly, normally required where crystal mismatches are encountered, is avoided. The invention also provides a means of final assembly which eliminates the introduction of physical strains upon the crystal elements which could otherwise result in undesirable variations in the electrical response of such crystals to applied signal voltages.